Dunno - different playstyles, I guess. Ghal has talked about how frustrating Legendary can be, and with the exception of the telepathic Wraiths (that can pinpoint you through any number of obstacles) and plain AI cheating (like yesterday's FRG Brute), I can't really see his point, either. I have a ton of fun on Reach's Legendary. (WAY more fun than H2's Legendary, for example.)
I'll take a crack at it.
I love playing Halo 1 and Halo 3 on Legendary. Part of the reason those games were tough yet fun was because while there were more enemies and they were tougher, the Chief wasn't reduced to a delicate flower to make it hard.
Take the game on Legendary Firefight I had the other night with Fyre and company. Beachhead, and everyone else is dead. I'm running from Jackals and Hunters. I snag rockets, and tag a Hunter with all four. He's not dead.
I run down to the waterfront and grab the Spartan Laser there, and scope in for the kill....and die. I was at full health! Not from a grenade. Not from a FRG.
From a Jackal. On Legendary, an overcharge shot from a Jackal strips yours shields and puts you deep in the red for heath. The Jackal followed up with a couple plasma shots, so he killed me in less than one second. One Jackal.
I don't find that kind of difficulty fun.
But there's more to it. Because I'm so fragile, Brutes and Elites can one-punch me to death, and it can take 3-4 hits to kill say, a Jackal.
Now factor in the base traits. If a plasma grenade gets tossed near me, I will try to avoid it. Only, my momentum prevents me from changing directions quickly, my base speed prevents me from moving fast enough when I do, and my low jump means I can't get out of the way in time, and the speed with which plasmas blow up means I die, despite reacting fast enough.
That means close quarters combat is out of the question. And since guys like Jackals can kill you instantly, that means Legendary on Reach is all about hunkering down and doing nothing but long range fights, hoping not to get hit because I'm such a fragile little thing. And that's not fun; Halo is fun to be because of all the options I have at my disposal, and Legendary in Reach strips a huge swath of those options away.
In Halo 3 Legendary, I would still rumble toe to toe with Brutes and other enemies, since they didn't kill me instantly. I still used the full combat sandbox because the game still encouraged it (and even required it - the range of the BR prevented super long range donging).
It felt, in short, more fair. Reach's Legendary does not feel fair to me, and I can't stand it.
Obviously, I can't explain why you have fun with it. I'm glad you do! But I think Legendary in Reach is really, really borked.